1. Field of the Invention
This invention relates to a process for removing nitrogen oxides. More particularly it relates to a process for removing nitrogen oxides, which is difficult to cause reduction in the catalyst activity due to volatile metal compounds contained in exhaust gases, with a good efficiency.
2. Description of the Related Art
A process for removing nitrogen oxides (NO.sub.x) contained in various exhaust gases by reducing the nitrogen oxides with ammonia (NH.sub.3) is simple in the aspect of the system and is carried out with a good efficiency; hence the process has been a main current of denitration process for exhaust gases from various fixed sources, including boiler combustion exhaust gas. The process requires the so-called denitration catalyst for promoting the reaction of NO.sub.x with NH.sub.3, and various inventions therefor have so far been made. Among such catalysts, those which have been currently and practically used are catalysts containing titanium oxide (TiO.sub.2) as a main component and having added thereto oxides and/or sulfates of vanadium (V), molybdenum (Mo), tungsten (W), iron, etc. represented by those disclosed in Japanese patent application laid-open Nos. Sho 50-51966/1975 and Sho 52-122293/1977. These catalysts are difficultly deteriorated even when sulfur oxides are contained in exhaust gases and exhibit high denitration properties; hence they are superior catalysts.
However, as to these catalysts, reduction in the catalyst activity due to volatile metal compounds formed mainly from mineral components contained in fuels, that is, oxides of selenium, tellurium, tharium, arsenic, etc. or reaction products thereof with alkali metals, alkaline earth metals, etc., has not been taken into consideration.
Thus, a new problem has been raised that when the above catalysts are used for denitrating exhaust gases having a tendency that the concentration of the above volatile compounds in exhaust gases has been increasing due to use in recent years of coal or petroleum containing a large quantity of mineral substances, the catalyst activity is reduced to a large extent.